The rapid development of electronic logic devices for data processing and communications systems is placing rigorous demands on electrical connectors. Increasing integration of solid state devices, combined with the need to increase the speed of data processing and communication systems, requires that connectors have higher densities, higher pin counts, and better electrical performance than in the past.
Conventional connectors have traditionally used male and female connectors. A typical male connector provides a number of rows of pins extending from a connector housing. The housing of a typical female connector provides a number of rows of receptacles. The pattern of receptacles is intended to match the pattern of pins on the male connector. When the male and female connectors are mated, the pins are forcibly inserted into the receptacles. When the connectors are unmated, the pins are forcibly withdrawn from the receptacles. The forces applied to the pins during mating and unmating require that the pins be thick enough or rigid enough to endure these forces without damage. Traditional connectors have, therefore, been relatively large so that the pins could be designed to withstand these forces.
Density and pin count are often viewed interchangeably, but there are important differences. The density refers to the number of contacts provided per unit length. In contrast, the number of contacts or pins that can reasonably withstand the mating and unmating forces is referred to as the pin count.
As more functions become integrated on semiconductor chips or on flexible circuit substrates and more chips are provided on printed circuit boards (PCBs), each PCB or flexible circuit must provide more inputs and outputs (I/Os). In addition, many system components are capable of operation at faster speeds than previously. The connectors used in high-speed board-to-board (including both PCBs and flexible circuits) communications may be treated like transmission lines in which crosstalk and noise become significant concerns. The electrical performance of high-speed board-to-board communications is, therefore, dependent upon the amount of crosstalk and noise introduced at the PCB interfaces.
Although, the invention and background of the invention are described in terms of connecting PCBs it should be understood that the invention should not be limited thereto. Specifically, any reference to PCBs herein is intended to include other circuit substrates such as flexible circuits.
Dimensional mismatches in conventional connectors also degrade the system's electrical performance. Specifically, the size and location of the connector pins and receptacles of the mating connector may differ resulting in an unstable connection. Moreover, if the pin and receptacle pattern of a male and female connector, respectively, differ or are slightly misaligned, the electrical interface provided by the connection may be impaired.
Density, pin count, and electrical performance are related to one another. Design factors should be balanced to optimize the connector in terms of its density, pin count and electrical performance. Density can be increased by decreasing the distance between contacts and by increasing the number of rows in a connector. Increasing the density may also increase the pin count because 1) more pins are available for mating and unmating, and 2) higher density reduces the linear tolerances per pin as mating and unmating forces are averaged over more pins. An increase in contact density may, however, adversely affect the electrical performance of the connector since crosstalk can increase by bringing the pins into closer proximity to one another. The addition of rows to increase the contact density may also increase the electrical path length of signals transmitted over the board-to-board interface thereby reducing the speed of the system and increasing the potential for noise.
The pin count is limited in part by the mechanical forces applied when the connector is mated and unmated. Some connectors have been designed to reduce these forces by providing contact elements that extend from both male and female connectors or from hermaphroditic connectors. Examples of such connectors are disclosed in U.S. Pat. No. 3,868,162, issued to Ammon on Feb. 25, 1975, and U.S. Pat. No. 5,098,311, issued to Roath et al. on May 24, 1992, respectively. (Contact connectors may be referred to as blade-on-beam connectors when one contact is static and the mating contact is compliant or beam-on-beam connectors when both of the mated contacts are compliant.) However, the pin count for contact connectors is limited by tolerances imposed on the contacts from the contacts of the mating connector. A balancing of the elasticity, maximum stress, and the dimensions of the contacts is required to provide an adequate normal force between the mated contacts for a stable electrical interface. Balancing of these factors may in turn affect density and pin count.
A contact connector functions by bringing metal contacts, that are typically attached to electronic subassemblies, together with a specified amount of force or compliance. The force requirements are, to some degree, dependent on the application, environment and surface finishes. However, it is generally accepted that under the best of conditions, with precious metal platings covering the contacts, a minimum contact force of about 35 grams is required.
Modern data processing and communications components use "mezzanine" or parallel board stacking arrangements in which the planar surfaces of printed circuit board assemblies are connected parallel to one another. In miniaturized systems, it is desirable to reduce the profile or height of the connectors that interconnect the printed circuit boards. State-of-the-art connectors presently have profile heights of about 3.5-4.0 mm, with linear contact spacings of about 1 mm. To accommodate the miniaturization of modern electronics it is desirable to have connectors with profile heights as low or lower than 2 mm with contact spacings of approximately 0.5 mm.
Another factor that is significant in connector design is cost. Generally blade-on-beam connectors merely provide a plurality of contacts attached to a housing. Thus, in miniaturized connectors, material costs are relatively small as compared to labor or conversion costs. Usually the most significant cost factor in the production of a contact connector is the assembly of the contacts to the housing.
There are three basic methods of assembling contact connectors:
1. Stitching contacts into plastic housings; PA1 2. Mass inserting contacts into plastic housings; and PA1 3. Molding a plastic housing around a strip of contacts. PA1 P.sub.min is the minimum normal force of the electrical contacts required to provide an electrical interface; PA1 L is the length of the electrical contacts; PA1 b is the width of the electrical contacts; PA1 h is the thickness of the electrical contacts; PA1 E is the elasticity of the electrical contacts; and PA1 S.sub.max is the maximum stress of the electrical contacts.
Each of these methods has many advantages and disadvantages, and the costs associated with each method can vary considerably depending on the connector design, program economics, product variations, etc.
According to the first method, contacts are individually stitched or pressed into a plastic housing. The housing is provided with preformed holes through which the contacts are inserted. This method requires individual handling of each contact causing the process to be time-consuming and relatively expensive. Moreover, individual insertion of contacts through the preformed insertion holes increases the risk that the contacts may not be securely embedded in the housing.
The second and third methods have a better potential for ultimately low costs, since they eliminate the separate handling of the individual contacts during the connector assembly process. These methods traditionally require that the contacts be stamped on their finished centers. This means that the center-to-center distance (referred to as "pitch") between contacts on the original stamped strip is the same as that required in the finished connector. In particular, a plurality of contacts attached by a detachable carrier strip is stamped out of a strip of conductive material forming a contact strip. The detachable carrier strip can be used to hold a plurality of contacts simultaneously.
In "mass insertion" systems, contacts are either latched or staked into the insertion holes of a plastic housing. The carrier detachable strip may then be removed from the contacts. Because of potential dimensional variations between the contacts and their respective spacing and the insertion holes, the contacts may not be securely embedded in the housing.
According to the third method, the strip of contacts are first placed in a mold. The mold is then injected with a plastic material which sets to securely embed a portion of the contacts in the housing. After the plastic has set, the housing can be removed from the mold and the detachable carrier strip detached from the contacts. Insert molding eliminates the need to handle individual contacts during the insertion process, thereby further reducing the processing cost. Both the connector pin count and the connector density may be limited by using insert molding since conventional insert molding processes produce connectors with only a single row of contacts.
The requirement of having contacts stamped on centers limits the potential width and thickness of the contact. Stamping guidelines indicate that it is not desirable to blank a strip of metal to produce contacts having their widths less than their thickness. If these guidelines are not followed, the contacts may be twisted or subjected to too much stress. Additionally, stamping punches designed to blank metal strips to provide narrower contacts than the thickness of the metal strip are fragile and difficult to maintain. Moreover, reducing the width and thickness of the contacts may reduce the electrical performance of the connectors since the compliance of the contacts can be expected to decrease as the width and thickness are reduced.
Single beam contacts normally require that they be firmly anchored in the connector, since the stresses that are generated as a result of the contact deflection usually are highest at the base of the contact in the area where the contact is attached to the connector housing. This is another reason that molding the housing around the contacts is desirable in miniaturized connectors--there is reasonable certainty that the contact base is securely supported by the connector housing.
Traditionally, the connector contacts for connecting printed circuit boards in a stacked arrangement have been designed to extend from the housing in a direction orthogonal to the planar surface of a printed circuit board. It has, therefore, been impractical or impossible to use traditional stamped on-center contact strips and insert molding, with existing materials and processes, to design a connector with contacts having both the requisite compliance to provide a stable electrical interface, and achieve the desired low connector profile height. For example, U.S. Pat. No. 3,193,793, issued to Plunkett et al. on Jul. 6, 1965 discloses a connector design in which a maximum contact area is provided to optimize the stability of the electrical interface while minimizing the connector height profile. However, the connector disclosed by Plunkett et al. provides these advantages by forfeiting high density, high pin count and the benefits associated with insert molding.
U.S. Pat. No. 5,083,696 issued to Kan et al. discloses pin holding devices for use in connecting printed circuit boards in a stacked arrangement. However, the pins are used to permanently connect boards in a stacked arrangement such that the connection of boards is not interchangeable without at least some damage to the pins.
Therefore, there is a need to provide a stable low profile connector having a substantially maximum pin count, density, and substantially optimal electrical performance that can be produced by a low-cost process, such as insert molding.